1. Field of the Invention
The present invention relates generally to solid-state imaging devices, and more particularly to a color image sensing system including three separate charge transfer devices packed therein.
2. Description of the Related Art
In the recent past, there have been several improvements in image sensing systems for pickup of color images. The image sensing systems may include a movie camera such as a video camera for use in the domestic or international television broadcasting stations. As the camera requires higher image quality, a pickup device employed therein becomes more critical in its photosensing performance. To meet such high-quality image pickup requirement, a color movie camera using three separate charge coupled device (CCD) image sensors are becoming widely used in the television broadcasting systems. The three image sensors function as a red-color sensor, a green-color sensor, and a blue-color sensor, respectively. In this regard, the camera is generally called the "three-chip color camera" in this field of technology.
Typically, an interline-transfer type image sensor or a frame-interline transfer type sensor is used as each of the CCD image sensors. The image resolution of the three-chip color movie camera is very important in the achievement of high-quality pickup images, since the resolution concerns directly to the basic pickup performance of each CCD image sensor. Until today, several techniques have been proposed in order to attain the required pickup performance for high-quality pickup images, in other words, to improve the physical and/or electrical resolution characteristic of the CCD image sensors.
As one of the resolution enhancement techniques for the CCD image sensors, it has been proposed to drive each image sensor having a matrix of charge packet storage cells (picture elements or "pixels") in such a manner that the carrier-storage (integration or accumulation) period of alternate arrays of cells in the matrix is different in time length from that of the remaining cell arrays. The technique is shown, for example, in Japanese Patent Application Disclosure (KOKAI) 63-209280 (1988). With the "field-inversion carrier read" technique disclosed therein, the carrier storage amount is reduced by forcing either the cell arrays on the even-numbered lines or those on the odd-numbered lines in the cell matrix to be shorter than ever before in the length of carrier-integration period. The vertical resolution of the CCD image sensor can thus be improved. However, this causes the photosensitivity to decrease. In other words, the prior art suffers from an inherent decrease in the sensitivity while the vertical resolution characteristic is enhanced, which may be based on the generally known "trade-off" between the sensitivity and the resolution of the CCD image sensors.